On Jul 28, 9:40*pm, John Hall wrote:
In article ,
*David Gartrell writes:

I was wondering how far ahead the GFS pressure, temperature & rainfall
charts can be relied upon. *The reason for the question is that we're
off on our Holidays to Switzerland in August and the Gfs charts seem to
be suggesting a much improved outlook which quite frankly i'm
desperately hoping is correct. *I had read somewhere that the charts up
to +240 can be considered fairly accurate but become rather fanciful
after that.

I think that's over-optimistic. The accuracy normally starts to tail off
after 5-6 days, and is usually quite low by 10 days (ie +240). Pressure
will be most accurate, because small inaccuracies in the derived
pressure pattern, and the consequent position of fronts, can have a big
impact on temperature and, in particular, rainfall.

You can get a better idea by seeing if the three main models, GFS, ECMWF
and UKMO are consistent with each other and from run to run. If the 06Z
GFS run is radically different from the 00Z, and then the 12Z is
different again, that suggests that the outcome is likely to be on a
knife-edge.
--
John Hall

* * * * * * "I don't even butter my bread; I consider that cooking."
* * * * * * * * * * * * * * * * * * * * * * * * * *Katherine Cebrian

I didn't read John's post before writing mine, honest ref!